In this study, a series of composite polydimethylsiloxane (PDMS)-polysulfone (PSF) membranes were prepared to investigate the influence of support layer structure on the mass transfer in pervaporation (PV). By varying polymer concentration in the casting solution and the evaporation time in the air, phase-inversion PSF membranes with different structures and resistances were obtained as the support layers. The results indicated that the support layer could dominate the PV performance of the composite membrane when it showed a significant resistance. An average parallel flow model was proposed to evaluate the influence of support layer structure on mass-transfer resistance in composite membranes. The inhomogeneous structure of the support layer could produce extra mass-transfer resistance when the resistance of the support layer could not be negligible. (C) 2009 Elsevier B.V. All rights reserved.